To allow you to track statistics for
different sized banners across all of your web pages, and

To show multiple ads on the same web
page.

When placing multiple ads on the same web
page, you must use a unique zone for each area that will be showing an ad.
If you don't you might see the same ad appearing twice. Below is a
sample web page showing one of many possible layouts. Note that each
zone is distinct, beginning with zone1 at the top and ending with zone 4.
Actual zone numbers will vary depending on how many you have added to your
web site.